In a legged mobile robot such as a biped mobile robot equipped with a plurality of legs, each leg is brought into contact with a floor through a ground-contacting face portion of a foot mechanism provided on a far end portion thereof. More particularly, the foot mechanism is a mechanism connected to a joint on the farthest end side of each leg (an ankle joint). The legged mobile robot moves by lifting and landing motions of each leg. More particularly, the lifting and landing motions are a repetition of motions that while at least one leg of a plurality of legs as a supporting leg maintains a foot mechanism of the supporting leg in a ground-contacting state, the other leg as a free leg lifts a foot mechanism of the free leg from its ground-contacting location into the air and moves the same, and makes contact with the ground on other ground-contacting location.
In such a legged mobile robot, when a ground-contacting face portion of a foot mechanism of the leg is brought into contact with the ground by the landing motion of each leg, a relatively great impact load (a transient floor reaction force) instantaneously acts through the foot mechanism of the leg. Particularly, when the legged mobile robot is moved at relatively high moving speed, motion energy of the leg in moments immediately before the foot mechanism of the leg makes contact with the ground is great, so that the impact load will be high. When this impact load is high, rigidity of each portion of each leg needs to be enhanced in order to resist the load, and furthermore, this will interfere with a size reduction and a weight reduction of each leg. Accordingly, a reduction (shock absorption) of such an impact load is desired.
As such a shock absorber, for example, the one that the present applicant proposed in Japanese Patent Laid-Open Publication No.5-305578 is known. In this shock absorber, a cylinder filled with hydraulic oil is provided at a heel of the foot mechanism, and a rod is extendedly provided from a piston slidable in this cylinder toward a bottom face side of the heel of the foot mechanism. A ground-contacting element widened in diameter in a mushroom shape is provided on a tip portion of the rod. Additionally, the piston is energized in a direction that the ground-contacting element projects to the bottom face side of the foot mechanism by a spring accommodated in the cylinder on the upper side thereof. Furthermore, in the piston, a flow passage that allows the hydraulic oil to flow between an upper chamber and a lower chamber thereof is drilled.
In the shock absorber configured in this manner, at the time of the landing motion of the leg, the aforementioned ground-contacting element makes contact with the ground and is pressed with the piston in a direction opposite to an energizing force of the spring. At this moment, while the hydraulic oil in the cylinder flows through the flow passage of the piston, the piston slides in a direction that the piston compresses the spring, and this allows the impact load during the landing motion of the leg to be reduced.
However, in such a shock absorber, in a landing state of the leg (a state of a supporting leg stage of the leg), since the ground-contacting element of the heel portion of the foot mechanism makes contact with the ground and is pressed against the energizing force of the spring, a floor reaction force constantly acts on the heel portion of the foot mechanism. Furthermore, in some gait types of the robot, the ground-contacting element may be still in contact with the ground immediately after the leg is lifted from the floor. Therefore, there are times when a lifting motion of the leg cannot be performed smoothly during the movement of the robot, and stumbling is caused. Additionally, in the landing state of the leg, since the floor reaction force constantly acts on the heel portion of the foot mechanism, the floor reaction force cannot be made to intensively act on a desired position of the foot mechanism (for example, tiptoe or the like) to secure posture stability of the robot. Therefore, the posture stability of the robot is apt to be impaired by a geometry of a floor, external force or the like.
Furthermore, in the shock absorber, as a result of the use of the hydraulic oil, weight of the shock absorber will be heavy, resulting in interfering with a weight reduction of the robot. Additionally, the ground-contacting element that makes contact with the ground during the landing motion of the leg can only move in a sliding direction of the piston (an axial center direction of the cylinder) and is a solid body. Consequently, the impact load acting on the ground-contacting element may act in a direction that crosses a movable direction thereof depending on a geometry of a floor, so that the impact load may not adequately be reduced, and damage of the shock absorber may be generated.
The present invention is achieved in the light of such a background, and it is the object to provide a landing shock absorber which can easily secure posture stability of a legged mobile robot while reducing an impact load during a landing motion of a leg of the robot, and further, which can be configured to be lightweight.